It is generally known to increase the density of porous bodies mainly consisting of SiO.sub.2 by heating the bodies in a furnace.
The major problem in this method is to prevent gas bubbles from being included in the glass during densification. It has therefore been already suggested to pass a softening front through the body while using an annular furnace. In this case heating of the densified body by convection cannot be avoided. (See, for example, German published Patent Application DE-OS No. 3 240 355). When this method is used, small gas bubbles or vacuum cavities (vacuoles) may also still be present after densification. It is therefore recommended in this method to ensure that the gas bubbles are filled with helium or hydrogen. In a subsequent thermal treatment the gases may diffuse away out of the glass. The vacuoles must then be driven out by a thermal treatment.
In this method the possibility exists for the glass to be heated at a high temperature for such a period of time that crystallisation of the glass may occur if the major constituent is SiO.sub.2. It is the object of the invention to provide a method of increasing the density of a porous pre-formed body of a material having a main constituent of SiO.sub.2 in which the formed transparent glass is not essentially heated.
A material having a major constituent of SiO.sub.2 is to be understood to mean in this connection a material which comprises an excess of SiO.sub.2 and further admixtures (dopants) in a quantity as is usual either to reduce or to increase the refractive index of silica glass by a percentage as is usual for the application of the glasses in optical fibers. The material is also to be understood to include a material which comprises only SiO.sub.2 including inevitable impurities.